Genetic variations occur in susceptibility to infection in Biomphalaria glabrata and infectivity in Schistosoma mansoni. Studies indicate at least 10 genetically different susceptibility types in B. glabrata. Exposures of B. glabrata have demonstrated at least three different S. mansoni strains of Puerto Rican origin. Isolations from Puerto Rican cases involved mixed infections, yielding two genetically different S. mansoni strains. A simple, dominant, pigment mutant in B. glabrata, occurring on a different chromosome from the gene determining albinism, results in coalescence of mantle spotting in wildtype and blackeye snails. Using labelled miracidia and autoradiography it has been shown that when a B. glabrata susceptible to one S. mansoni strain but resistant to another is exposed to both, either in series or at the same time, the host reaction is strain specific. Enzyme studies have shown increases in lysozyme in hemolymph and headfoot extract shortly after miracidial penetration, and the increase is greater in resistant than in susceptible snails. BIBLIOGRAPHIC REFERENCES: Richards, C.S.: Schistosoma mansoni: Susceptibility reversal with age in the snail host Biomphalaria glabrata. Experimental Parasitology, 42: 165-168, 1977. Lie, K.J., Heyneman, D., and Richards, C.S.: Studies on resistance in snails: Interference by nonirradiated echinostome larvae with natural resistance to Schistosome mansoni in Biomphalaria glabrata. Jour. Invert. Pathol. 29: 118-125, 1977.